© 2015. Published by The Company of Biologists Ltd.

Body appendages fine-tune posture and moments in freely

manoeuvring fruit flies

Ruben Berthé and Fritz-Olaf Lehmann*

Department of Animal Physiology, University of Rostock, Germany

*Address for correspondence:

Albert-Einstein-Str. 3, 18059 Rostock, Germany

Phone +49 381 498 6301

Fax +49 381 498 6302

email: fritz.lehmann@uni-rostock.de

Key words: aerodynamics, body appendages, Drosophila, free flight, insect flight,

locomotion, moment control, moments of inertia, posture stability

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http://jeb.biologists.org/lookup/doi/10.1242/jeb.122408Access the most recent version at J Exp Biol Advance Online Articles. First posted online on 7 September 2015 as doi:10.1242/jeb.122408http://jeb.biologists.org/lookup/doi/10.1242/jeb.122408Access the most recent version at

First posted online on 7 September 2015 as 10.1242/jeb.122408

ABSTRACT

The precise control of body posture by turning moments is a key to elevated

locomotor performance in flying animals. Although elevated moments for body

stabilization are typically produced by wing aerodynamics, animals also steer using

drag on body appendages, shifting their centre of body mass, and changing moments

of inertia owing to active alterations in body shape. To estimate the instantaneous

contribution of each of these components for posture control in an insect, we three-

dimensionally reconstructed body posture and movements of body appendages in

freely manoeuvring fruit flies Drosophila melanogaster by high speed video and

experimentally scored drag coefficients of legs and body trunk at low Reynolds

number. The results show that the sum of leg- and abdomen-induced yaw moments

dominates wing-induced moments during 17% of total flight time but on average is

7.2-times (roll, 3.4-times) smaller during manoeuvring. Our data reject a previous

hypothesis on synergistic moment support, indicating that drag on body appendages

and mass-shift inhibit rather than support turning moments produced by the wings.

Numerical modelling further shows that hind leg extension alters the moments of

inertia around the three main body axes of the animal by not more than 6% during

manoeuvring, which is significantly less than previously reported for other insects. In

sum, yaw, pitch, and roll steering by body appendages likely fine-tunes turning

behaviour and body posture, without providing a significant advantage for posture

stability and moment support. Motion control of appendages might thus be part of the

insect's trimming reflexes that trim out imbalances in moment generation owing to

unilateral wing damage and abnormal asymmetries of the flight apparatus.

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INTRODUCTION

Locomotion during migration, territory defence, routine commuting, foraging, and

escape behaviours is vital to the reproductive effectiveness and survival of many

animals (Alexander, 2003). A critical task during locomotion is the control of body

posture, which is tightly linked to the production and control of locomotor forces for

propulsion and the corresponding moments for turning control (Ellington, 1984a;

Ellington, 1984b). While locomotion involving contact with solid ground benefits

from static stability, flying animals achieve body stability and thus flight path control

with the help of aerodynamic friction of their environment. Active manoeuvring and

directed aerial descent such as gliding thus depend on the animal’s ability to fine tune

the benefits of three distinct, major physical mechanisms for posture control:

aerodynamic lift and drag on wings and body (Pennycuick, 1968; Pennycuick, 1971),

active control of the centre of body mass and thus distance to aerodynamic force

vectors (Cook and Spottiswoode, 2006; Ellington, 1984a), and modifications of the

body’s moments of inertia by active changes in body shape (Libby et al., 2012).

The control of moments of inertia is highly effective even in terrestrial animals, for

example, during falling and jumping, in which rats maintain an upright body posture

by twisting their entire body (Laouris et al., 1990) and geckos and lizards by beating

their heavy tails (Jusufi et al., 2008; Libby et al., 2012). In preparation of targeted

jumps, moreover, flightless mantis generate a controlled whole-body spin by

adjustment of their center of mass (Burrows et al., 2015). Since active modification in

body shape leads to a change in moments of inertia, this mechanism also enables

astronauts to control their body orientation without air friction (Kane and Scher, 1970;

Kulwicki et al., 1962). By contrast, gliding and actively flying animals typically

control moments and posture by alterations in the wings’ lift and drag characteristics

(Ellington, 1984a; Ellington, 1984c; Ellington, 1991). Although wing aerodynamics

predominantly determines moments for posture control, an increasing number of

studies highlights that aerial steering is effectively supported by the aerodynamics of

body appendages such as legs and abdomen. Arboreal ants and other wingless, gliding

hexapods, for example, effectively control their aerial descent from tree to tree by

steering with their hind legs (Yanoviak et al., 2010) and lateral cerci, respectively,

(Yanoviak et al., 2009). If the hind legs are cut, the tree trunk landing success is

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severely attenuated between 35 and 60%. Leg steering is also of great importance for

drag control in birds. Depending on body posture, pigeons and griffon vultures, for

example, may increase their total body drag coefficients during forward flight by

factors of approximately 2 and 3, respectively, depending on the extension of their

feet (Pennycuick, 1968; Pennycuick, 1971). As a consequence, feet adduction in birds

during smooth weather conditions leads to an increase in gliding distance, while

during manoeuvres the feet appear (Pennycuick, 1960).

The vast majority of previous studies on the significance of body appendages for force

and moment support in actively flying animals were conducted in insects such as the

small fruit fly (Götz et al., 1979; Zanker, 1988b), the house fly (Zanker, 1991), orchid

bees (Combes and Dudley, 2009) and moths (Cheng et al., 2011; Hedrick and Daniel,

2006). Early studies on various freely flying insect species suggested that leg steering

and shifting the insect’s centre of body mass support wing-induced moments during

manoeuvring (Ellington, 1984d). This hypothesis was further investigated under

visual stimulation mimicking yaw turns, during which tethered flying flies bend hind

legs and abdomen in the horizontal to the inner side of the intended turn (Götz et al.,

1979; Zanker, 1988a; Zanker, 1988b; Zanker, 1991). Visual stimulation mimicking

body pitching, by contrast, leads to bending of the abdomen in the vertical, with

upward bending during upward motion of the visual pattern (Dyhr et al., 2013; Frye,

2001; Hinterwirth and Daniel, 2010). Mathematical models of the latter behaviour

demonstrated that abdominal steering is, at least to some extent, sufficient to maintain

body posture in the hawkmoth (Cheng et al., 2011; Dyhr et al., 2013; Hedrick and

Daniel, 2006). Besides vision, some insects such as desert locusts also bend their

abdomen in response to changing air flow conditions (Arbas, 1986). It has been

suggested that this behaviour mimics an aerodynamic rudder that helps the animal to

orient into the direction of wind during flight (Camhi, 1970a; Camhi, 1970b).

Here we show the significance of leg and abdominal steering on moments and body

posture in freely manoeuvring fruit flies Drosophila, estimating instantaneous

moments owing to wing motion, hind leg- and abdomen posture around the yaw,

pitch, and roll body axes, respectively. For this investigation, we (i) employed high-

speed video to three-dimensionally reconstruct the animal’s body posture and

extension angles of legs and abdomen during various flight manoeuvres, (ii) measured

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drag coefficients of hind legs and the body trunk in a wind tunnel and (iii) derived

turning moments from a numerical approach. In contrast to previous hypotheses, our

data suggest that body appendages mostly attenuate rather than enhance wing

flapping-induced moments. We argue, moreover, that owing to its small contribution

to total moments, leg- and abdomen-induced moments should be considered as

control systems for fine control rather than systems that significantly enhance the

production of moments during extreme flight manoeuvres.

RESULTS

Steering by wings and body appendages

Our recorded flight sequences cover various flight manoeuvres of female fruit flies

Drosophila melanogaster including rectilinear horizontal flight, ascending and

descending flight, shallow turns and rapid flight saccades, in which animals rotate

with angular peak velocity of up to 1074 deg s−1 (mean, 504 ± 49.1 deg s−1, N = 6

saccades) around the vertical yaw axis (Fig. 1A and B). The mean of all data points

within each flight sequence that fell within the top 1% maximum of horizontal

(vertical) velocity values was 0.26 ± 0.13 m s−1 (0.15 ± 0.11 m s−1), while the mean of

all data samples amounts to 0.21 ± 0.12 m s−1 (0.11 ± 0.11 m s−1, N = 81 flight

sequences). The free flight analysis highlights that body yaw, pitch, and roll angles

continuously change during manoeuvring flight, exhibiting small changes of several

degrees during straight flight and pronounced changes in body posture of up to

approximately 30 deg roll angle during rapid turns (Fig. 1A and C). The normalized

histograms of posture angles demonstrate a mean body pitch of approximately 30.6 ±

9.7 deg and a mean roll angle near zero of −0.6 ± 9.4 deg (N = 24,579 video frames)

during flight.

Free flight manoeuvres in Drosophila are accompanied by extensive movements of

hind legs and the abdomen. On average, the animals bend their abdomen vertically

downward by approximately 10.1 ± 5.3 deg with respect to the longitudinal body axis

at a mean horizontal deflection angle of the abdomen close to zero (2.0 ± 3.5 deg).

Abdomen steering angles typically vary −15 to 15 deg around its mean value (Fig.

1D). By contrast, leg extension angles (right-minus-left) vary between −35 and 35

deg, with mean vertical and horizontal leg extension angles of 108 ± 15.1 deg and 112

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± 13.7 deg, respectively (Fig. 1E and F). For a definition of angles refer to Materials

and Methods section.

To evaluate the contribution of moments caused by changes in aerodynamic drag on

both body trunk (head, thorax, abdomen, fore- and middle legs) and hind legs to wing

flapping-induced moments during flight manoeuvres, we systematically analyzed the

magnitude and coherence of the three components for total moment control (Fig. 2B

and C). Figure 2 shows the moment components for yaw (Fig. 2E), pitch (Fig. 2F) and

roll (Fig. 2G) of the flight manoeuvre in Fig. 2A. The data suggest that aerodynamic

drag- and mass shift-induced moments are only small fractions of total moments

acting on the fly body (Fig. 2D-G). We derived the total moments from the changes in

body posture and a numerical framework (cf. Materials and Methods). The top 1%

maximum, absolute total moment of all data around the yaw, pitch, and roll axes was

11.2, 10.2 and 26.8 nN m, respectively (N = 246 samples). On average, the individual

contributions of hind leg-induced moments were approximately 41.3- (yaw), 11.2-

(roll), and 78.3-times (pitch), and contributions of the body trunk approximately 7.9-

(yaw) and 3.8-times (pitch) smaller than total moments produced by wings, hind legs,

and trunk (Table 1). Further calculations of moments using static, non-moveable legs

and abdomen (MI = 0, ζV = 105 deg, ζH = 143 deg, ψV = 0 deg, ψH = 0 deg) with

average posture show that "static" absolute MD* differs by approximately 3.4 pN m

(46%, yaw), 16.7 pN m (47%, pitch), and 3.7 pN m (21%, roll) from MD produced by

moving legs and abdomen (single animal, Fig. 3). Considering only static legs

(abdomen), mean MD* differs by approximately 10.0 ± 9.5 pN m (6.4 ± 8.7 pN m,

yaw), 19.4 ± 18.9 pN m (29.8 ± 21.2 pN m, pitch), and 15.0 ± 14.9 pN m (7.1 ± 8.0

pN m, roll) from MD (N = 81 flight sequences).

The magnitude of turning moments depends on the product of moment arm and

aerodynamic force, and thus on local air velocities on legs and abdomen. These

velocities result from three distinct kinematic components: the body’s translational

motion (for-, up-, and sideward), its rotational motion around the three main body

axes, and active leg motion relative to the body (Fig. 1A, Table 1). For completeness,

we also considered the wing's induced flow (downwash) on legs and body, which is

outlined in more detail in a section below. The moments around the fly's main axes

(yaw, pitch, roll) owing to translational body motion are shown in Fig. 2H, rotational

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motion is shown in Fig. 2I, active leg motion in Fig. 2J, and moments owing to

induced flow from the beating wings in Fig. 2K. The data suggest that body

translation predominately determines moment control in fruit flies cruising at mean

forward speed of 253 ± 137 mm s−1, while the contribution of rotational body motion

is small owing to small angular speeds and a decreasing velocity gradient from leg tip

to base (Table 1). Mean tarsal velocity of the hind legs due to body rotation amounts

to only approximately 13.4 ± 14.7 mm s−1 and the 1% maximum does not exceed

107 mm s−1. The velocity component owing to active leg motion (7.04 ± 7.99 mm s−1)

is almost negligible and 36- and 2-fold smaller than the velocities induced by body

translation and rotation, respectively. We obtained similar results for active translation

motion of the abdomen (3.77 ± 1.64 mm s−1) that compares to mean body rotation-

induced velocity of 6.75 ± 4.60 mm s−1.

Although our data indicate that drag-based steering by hind legs is likely to be 7.6-

fold more effective than drag-based steering by the abdomen, the significance of body

appendages for moment control during manoeuvring flight is limited (Fig. 4A and B).

An analysis on the relative contribution of drag-, mass shift-, and inertia-induced

moments for yaw control shows that the sum of all three moment components exceeds

wing flapping-induced moments in only 16.8% (22.9% for roll) of total flight time

(Fig. 4C and D). Drag-based moments alone are higher than wing flapping moments

in 1.7% (yaw) and 0.9% (roll) of the flight time (Fig. 4E and F). In other words:

during half of total flight time, the total moments produced by body appendages for

yaw and roll amount to only 2.0% and 3.3% of wing-induced moments, respectively.

Coherence of steering moment components

In contrast to previous tethered flight studies on vision-induced yaw steering in

Drosophila, hind leg and abdomen deflection is more variable in free flight. Our data

even suggest that leg and abdomen deflection is broadly independent from total, wing

flapping-dominated moment control. We found that only in 51.8% of total flight time,

the sign of the instantaneous, relative (left-minus-right), horizontal leg extension

angle H (cf. Materials and Methods section) equals the sign of the total (sum of wing-

and body-induced) yaw moment MM. At these instants, the unsigned difference

between left and right horizontal leg extension angle is 11.2 deg ± 8.41 deg (mean,

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N = 81 flight sequences). At times (48.2% flight time) at which relative leg extension

angle and total yaw moment had a different sign, the unsigned difference between left

and right H is 10.1 deg ± 8.17 deg (mean, N = 81 flight sequences). For an additional

statistical test, we dichotomized the data with +1 (–1) that represents times at which

leg extension and yaw moments had equal (opposite) direction. We found no

significant difference between the two means for all flight sequences (0.03 ± 0.35,

Wilcoxon test, p = 0.14, N = 81). Statistical comparison on the relationship between

yaw moments and horizontal abdomen bending yielded a similar result: times with

equal sign occurred during 51.2% total time without a statistical difference from zero

for the dichotomized data (0.02 ± 0.34, Wilcoxon test, p = 0.16, N = 81).

To further investigate the relationship between the various sources of body moments,

we conducted moment coherence and correlation analyses on yaw and roll (Figs 5 and

6). The moment coherence analysis suggests that the sign of wing flapping- and body

drag-induced yaw moments is opposite during approximately 63% (yaw; Fig. 5A) and

65% (roll) of total flight time. This means that leg-based steering inhibits rather than

supports turning moments produced by wing flapping. The result is also consistent

with findings from a correlation analysis on the temporal relationship between wing

flapping- and body drag-induced moments of each flight sequence. The latter analysis

shows significant, negative correlation coefficients of –0.45 ± 0.36 and –0.54 ± 0.36

(Pearson tests, N = 73 and 74 significant correlations with p < 0.05) for yaw (Fig. 5D)

and roll, respectively. Corresponding coherence and correlation values between wing

flapping- and mass shift-induced yaw moments are shown in Fig. 5B and E and data

for the relationship between wing flapping- and inertia-induced yaw moments in Fig.

5C and F. For completeness, Fig. 6 shows the remaining dependencies between the

four sources of moment production (MW, MD, MS, and MI). For statistical evaluation of

the data, we tested the presented mean correlation coefficients against zero. The test

yielded no significant differences of coefficients calculated for MW vs. MS (coefficient

for roll, –0.01 ± 0.48; Wilcoxon-test, p > 0.99, N = 64) and MW vs. MI (coefficient for

roll, 0.12 ± 0.49; Wilcoxon-test, p = 0.09, N = 58), whereas all other correlation values

for yaw and roll moments were significantly different from zero (Wilcoxon-test,

p < 0.01, Figs 5 and 6).

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The significance of induced flow

In the following section, we address the potential significance of the wing's

downwash on moment control by body trunk and hind legs. Since wings, legs and

abdomen are mechanically linked, and the distance between the wings' stroke plane

and hind legs is small, we excluded downwash-induced moments from the analyses in

Figs 2 - 6, including table 1. This is reasonable because body lift production by wing

flapping should decrease with increasing downwash-induced drag on legs and

abdomen. To maintain weight support under these conditions, the animal must

moderately increase its total body lift production, which in turn balances the moments

produced by downwash-induced drag. However, at least to some degree, the wings'

downwash dissipates after its acceleration at the stroke plane, losing kinetic energy

and thus altering the efficacy of momentum transfer from the wings to body and hind

legs. The downwash-induced moments thus depend on the distance between wings

and body appendages (legs, abdomen) that varies during manoeuvring flight. In the

following section, we evaluated the potential contribution of downwash to moment

balance by estimation of instantaneous downwash velocity derived from body mass,

instantaneous vertical body motion, and using actuator disc theory (cf. Materials and

Methods section).

We considered downwash at various strength and compared the resulting moments for

yaw, pitch, and roll with our previous approach (Figs 2-4). Figure 7A shows a time

trace of downwash-induced yaw moments (grey) of the flight sequence in Fig. 2A.

We subsequently added fractions of these downwash moments to total drag-induced

yaw moments MD, in steps of 10% (coloured, cf. MD in Fig. 2E). To determine the

maximum potential effect of downwash on MD, we calculated means of the unsigned

top 1% maximum values of all 81 flight sequences (pooled data set, N = 24,596

measurements). These values are approximately 4.7- (yaw), 7.6- (pitch), and 5.3-times

(roll) larger than the means in table 1. The results are shown in Fig. 7B for yaw, in

Fig. 7C for pitch, and in Fig. 7D for roll moments. Depending on downwash-induced

moment, yaw moments change by approximately 21% and have a local minimum at

40% downwash. Pitch moments linearly increase with decreasing downwash by a

factor of approximately 2.3, while roll moments appear to be widely independent of

downwash-induced moments. Small effects were also obtained for the frequency of

moment ratio MD by MW (yaw, Fig. 7E). Downwash-induced moments may also

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change the outcome of the correlation analysis. In two cases, the correlation

coefficients decrease with increasing consideration of downwash-induced moments.

This relationship is shown in Fig. 7F for coefficients of drag-induced yaw moments

MD vs. MS (blue) and MD vs. MW (red). By contrast, the correlation coefficient of MD

vs. MI (yaw, green) is not affected by downwash and remains close to zero. We

obtained similar results for roll moments (data not shown).

The significance of body posture on moments of inertia

Moment control in insects might also benefit from changes in moments of inertia

around the three main body axes owing to the active control of legs and abdomen (see

Introduction section). To investigate the contribution of active motions, we

numerically modelled the insect body as a solid cylinder with the mass of an average

fruit fly and the hind legs as cylinders using typical lengths, diameters and masses of

corresponding leg segments (cf. Materials and Methods section, Fig. 8A, Table 2). We

first calculated the dependency of yaw, pitch and roll moments of inertia from body

pitch angle in a model fly in which hind leg coxae and femurs are in a fixed position

relative to the thorax, and without tibiae and tarsi (truncated flies, Fig. 8B). The data

suggest that moments of inertia (yaw, roll) in fruit flies may change 9-fold from

approximately 0.11 to 9.9 ng m2 depending of body pitch angle. By contrast, the

moments of inertia owing to mass motion of the hind leg tibia and tarsi are

comparatively small compared to the moments derived from truncated flies,

amounting up to an increase in moments of approximately 5% in yaw (Fig. 8C), 0.8%

in pitch (Fig. 8D) and 6% in roll (Fig. 8E).

For comparison, we also evaluated the maximum instantaneous increase of moments

of inertia that a fly might reach during flight by mass motion of its legs, modelling

truncated flies with tibia and tarsi but empirically derived hind leg extension angles

that produced maximum moments of inertia. These data are plotted as red lines in Fig.

8C-E and confirm the small benefit of leg steering for the control of moments of

inertia in fruit flies. On average, the theoretical prediction differs less than 1%

increase in moments of inertia from data based on the kinematic reconstructions of leg

angles of tibia and tarsi (black, Fig. 8C-E).

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DISCUSSION

Steering by wings and body appendages

Our recorded flight sequences show a broad spectrum of different flight behaviours in

freely flying fruit flies, ranging from straight flight, sharp turns, backward and

sideward flight to pronounced changes in flight altitude (Figs 1 and 2). The three-

dimensionally reconstructed body posture, leg extension angles and abdomen bending

highlight active components for body stability and directional control. This is evident

from the elevated correlation coefficients between horizontal abdominal bending and

leg extension angles (Pearson test, R = 0.69), significant correlation coefficients

between wing flapping- and drag-induced yaw moments, and the correlation between

wing flapping- and inertia-induced yaw moments (Fig. 5A and D). Our findings are

also consistent with previous experiments on abdomen control in tethered flying fruit

flies (Zanker, 1987). The latter study also demonstrated that abdominal movements

may not be caused by flow generated from wing flapping. Moreover, table 1 and Figs

2 and 4 show that the various contributions of hind legs and abdomen to total

moments are small and typically restricted to times at which wing flapping-induced

moments are small. Our moment ratios analyses suggest that steering by hind legs and

abdomen dominates moment control in only approximately 17% of total flight time

(Fig. 4) and drag-induced moments are smallest compared to all other moment-

generating mechanism in fruit flies (Table 1). Thus, in analogy to ruddering of

animals in water and air, the small contribution of drag owing to the legs’ proper

motion rejects the idea that fruit flies may produce elevated moments by paddling

movements of the hind legs (Ristroph et al., 2011).

As outlined in the Materials and Methods section, the estimation of wing flapping-

induced moments is crucial in our analysis on the relative contribution of abdomen-

and leg induced moments to total moments. Thus, we here compare our estimated

wing flapping-dominated total moments with the moments measured in tethered

studies of Drosophila flying under various flight and visual feedback conditions.

Table 1 shows that moments around the fly's yaw, pitch, and roll axes peak at

approximately 11.2, 10.2, and 26.8 nN m. In tethered fruit flies flying under open-

loop visual feedback conditions, for comparison, yaw varies between 3 and 8 nN m

(Götz et al., 1979; Heisenberg and Wolf, 1979; Heisenberg and Wolf, 1988; Mayer et

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al., 1988) with peak values of 17 nN m (Tammero et al., 2004), roll varies between 3

and 20 nN m (Blondeau and Heisenberg, 1982; Sugiura and Dickinson, 2009) and

pitch may reach up to 6 nN m (Blondeau and Heisenberg, 1982). Computational fluid

dynamic modeling using free flight kinematics of fruit flies performing a saccadic

turn reported a peak yaw moment of 2.0 nN m (mean approximately 1.1 nN m,

Ramamurti and Sandberg, 2007) and peak yaw moment derived from a robotic wing

mimicking a saccadic turn amounted to approximately 1.9 nN m in the same species

(Fry et al, 2003). These values are broadly similar to the values shown in table 1,

which gives credence to the method used in the present study for estimation of total

moment.

Coherence of steering

Previous studies on flight control in tethered flies suggest that hind leg steering during

optomotor yaw response reinforces wing flapping-induced moments by strongly

increasing (decreasing) the leg extension angle on the inner (outer) side of an intended

yaw turn (Zanker, 1988a; Zanker, 1991). The latter studies proposed that this synergy

increases the fly's agility and manoeuvrability, which may in turn increase survival

rate during aerial predation by dragonflies (Combes et al., 2012a). Our data reject the

above hypothesis for unrestrained flying fruit flies. We found no preference for the

idea that leg extension and abdomen bending angles coherently support turning

direction of the animal. In contrast to leg kinematics, we even found that sequence-

averaged yaw moments owing to drag on legs and abdomen significantly inhibit than

support wing flapping-induced moments (negative correlation coefficient in Fig. 5D),

which might enhance posture stability. The same holds for mass shift-induced

moments, suggesting a synergistic function of motor pathways to hind legs and

abdomen (Fig. 5E). However, counter moments during yaw turning might also result

from an increase in local velocities at the hind leg on the outer side of a flight curve,

despite its smaller leg extension angle. Data indeed show that an increase in rotational

velocity of the body is positively correlated with an increase in hind leg velocity on

the outer side of a turn (Pearson correlation: R = 0.77). While the above mechanism

might thus passively restrict angular velocity in turning flight owing to aerodynamic

damping, flies actively shift their centre of mass (abdomen bending) to the body side

that counteracts moments generated by wing flapping. The latter mechanism is

independent from angular turning rate because it solely relies on changes in the length

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of the moment arm between the fly's centre of mass and the centre of flight force at

mid up-/down stroke (Fig. 2C).

The apparent synergy between drag- and mass shift-induced moments and their

attenuation on wing flapping-induced moments is also supported by its positive

correlation coefficients during yaw and roll control (Fig. 6A and D, yaw). This result

is consistent with previous findings on the coherence of leg and abdominal

movements in the tethered housefly (Zanker, 1991). The latter study demonstrated

that hind legs and abdomen move in-phase and in the same direction during vision-

controlled flight. However, our finding that positive (yaw, 51%; roll, 47%) and

negative moment coherence (yaw, 49%; roll, 53% flight time) occur with

approximately same frequency suggests a highly flexible system with quite

independently acting system components for moment control.

Significance of induced flow

This study considered induced flow (wing downwash) in detail because of its

unsettled contribution to drag-induced moments. Wings and legs are mechanically

connected and thus any increase in downwash-induced force pushing legs and body

downward in the vertical attenuates body lift production by wing flapping via the

mechanical link. After its initial acceleration, downwash velocity and vorticity are

thought to increasingly cease with increasing distance from the wings' stroke plane,

owing to the viscous forces of the surrounding air. However, owing to the small

distance of less than a millimeter between stroke plane and legs, this effect is assumed

to be small. The different lengths of moment arm for wing flapping- and drag-induced

moments do not allow simple predictions of moment control by changes in downwash

velocity. We tackled this problem by calculation of moments, in which we considered

downwash as an intervortex stream with uniform velocity but various strengths. Our

results in Fig. 7 suggest that induced flow alters total moments depending on the

rotational axis: roll moment is rather independent of induced flow and changes only

up to 5%, while yaw moments may potentially change up to 22% and pitch moments

up to 56% of total moment. Despite the pronounced percentaged changes,

consideration of moments owing to induced flow does not alter the main result of this

study, although these changes might matter in times at which wing-induced moments

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are relatively small. Collectively, the exact contribution of downwash to body posture

and turning control remains somewhat unclear and requires further investigations.

Significance of body posture on moments of inertia

An alternative benefit why insects actively control legs and abdomen during

manoeuvring flight resides in the associated changes in moments of inertia around the

three body axes. Since moments of inertia depend on mass distribution, any changes

in vertical and horizontal position of legs and abdomen may potentially modulate this

measure. It has previously been shown that orchid bees flying in turbulent air laterally

extend their hind legs. This increases drag by approximately 30% but also moments of

inertia around the animal's roll axis by up to 53%, which in turn should enhance flight

stability (Combes and Dudley, 2009). Owing to the smaller hind leg mass in fruit flies

of approximately 9.42 g or 1.2 ± 0.3% body mass compared to orchid bees (5.9%

body mass), the changes in moments of inertia are comparatively small: hind leg

motion in Drosophila alters moments of inertia of not more than 6% (Fig. 8).

Moreover, compared to a model fruit flies without hind leg tibia and tarsi, moments of

inertia during yaw, pitch and roll steering may not increase more than 8% of the

moments of inertia of the body trunk. The largest benefit of hind leg control in fruit

flies is on roll stability, which is consistent with data obtained from the orchid bee.

Consequently, leg extension in small insects such as the fruit fly appears to be of little

significance for posture stability but, nevertheless, might slightly enhance posture

stability of animals flying under turbulent environmental conditions that require

elevated steering performance.

Conclusions

This study shows how freely flying fruit flies control their flight path by synergistic

action of three independently working motor control systems, enhancing the degrees

of freedom for heading and posture control. In general, turning moments produced by

wing motion dominate both: moments produced by aerodynamic drag on body and

hind legs and positional changes of the fly's centre of mass by movements of hind legs

and abdomen. Their small contribution suggests that fruit flies broadly employ hind

legs and abdomen to fine-tune moments for flight rather than to produce large

moments required during flight saccades or optomotor responses. The latter highlights

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the potential importance of leg and abdominal steering for aerial manoeuvring during

straight flight. Nevertheless, while maximum body drag-induced yaw moments (0.19

nN m) corresponds to a unilateral wing stroke equivalent of only approximately 0.7

deg amplitude, maximum alteration in positional change of centre of body mass (2.6

nN m) converts into pronounced unilateral change in stroke amplitude of 8.8 deg

(Hesselberg and Lehmann, 2007). Alternatively, motion control of body appendages

might be part of the insect's trimming reflexes to trim out bilateral imbalances in

forces and moments during flight. These imbalances may result from unilateral

aerodynamic effective changes such wing damage, abnormal asymmetries of the flight

apparatus, and an imbalance in muscle mechanical power output and control for wing

motion (Bender and Dickinson, 2006; Hesselberg and Lehmann, 2009). In this regard,

legs in insects are aerodynamic rudders, similar to those that correct for the counter

torque from the propeller in airplanes.

Seen in a larger context, drag-based leg control in flying Drosophila appears similar

to the function of middle and hind legs in apterygote hexapods such as wingless

gliding ants for aerial manoeuvrability and gliding performance (Yanoviak et al.,

2009; Yanoviak et al., 2010). From an evolutionary point of view, fruit flies might

thus have inherited leg and abdominal motor control systems from their wingless

ancestors. The small benefit of leg control in Drosophila on moments of inertia,

however, runs counter to the idea that leg steering has primarily evolved as a

mechanism to enhance posture stability. In this respect, the male orchid bee might be

an exception because its hind leg tibia is greatly enlarged compared to other insect

species in order to collect scents (Combes and Dudley, 2009).

MATERIALS AND METHODS

High-speed video recording inside a free-flight arena

The flies were scored in a free flight arena, allowing automated recordings of body

posture, abdominal bending and hind leg motion using three-dimensional high speed

video (Shishkin et al., 2012, Fig. 9). To stimulate 0.1 g fluorescence dye markers on

the fly (Fig. 9B and C), we flashed UV-light emitting diodes with 60 s short voltage

pulses. Position accuracy of the video-tracked markers was within ±30 m and

images were recorded inside a volume of 20 mm width × 20 mm length × 25 mm

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height at 3500 Hz frame rate (Hedrick, 2008). We scored 81 flight sequences of 14

female, 3–5 day old wild-type Drosophila melanogaster (Canton S) with an average

body mass of 1.26 ± 0.04 mg. Total flight time of all analysed sequences was 7.03 s,

with individual sequences ranging from 15.7 ms to 355 ms, and mean ambient

temperature was 23.6 °C.

Positional reconstruction of leg segments

We simplified the reconstruction of leg extension angles, performing pre-tests on leg

movements in tethered flying flies (N = 7, Fig. 10). A conventional infra-red video

camera recorded the animal from lateral (V, vertical angle) and the bottom (H,

horizontal angle), while the fly changed wing and leg motion in response to a visual

stimulus. We scored the angles between (i) femur and tibia, , (ii) tibia and tarsi, ,

(iii) femur and a connection line through the femur-tibia joint and the fifth tarsal

segment, , and (iv) femur and longitudinal body axis, . From these angles, we

calculated mean angles and variances (N = 10 images of each fly and camera view)

that yielded: 74.2 ± 36.7 deg (V), 125 ± 18.3 deg (H), 128 ± 25.9 deg (V), 161 ±

10.5 deg (H), 105 ± 27.5 deg (V), 143 ± 11.9 deg (H), 58.4 ± 2.4 deg (V), and 45.9

± 4.8 deg (H).

These measurements and subsequent correlation analyses suggested: (i) the small

standard deviations of angle κ indicate that coxa and femur move only little during

steering, thus was considered to be a constant and the tibia-femur joint position

determined from thoracic fluorescent markers; (ii) the high standard deviation of

indicates that the animal predominately alters this angle during leg steering; (iii) angle

significantly depends on angle (Pearson test, vertical, p < 0.001, R = 0.98, N = 70;

horizontal, p < 0.001, R = 0.66, N = 422); (iv) angle V linearly depends on V

(Pearson test, p < 0.001, R = 0.82, N = 70); and (v) angle H is not linearly correlated

with H and thus considered to be a constant (Pearson test, p = 0.22, R = 0.06,

N = 422). From the leg extension angles in free flight and linear regression analyses

on leg angles in tethered animals, we determined remaining angles and leg positions

using the equations,

δV deg = 1.39 ζV deg − 68.62 deg, (1)

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δH deg = 0.87 ζH deg + 10.75 deg, (2)

and

εV deg = 0.77 ζV deg + 46.90 deg. (3)

Estimation of aerodynamic drag on legs and body trunk

Body appendages and thus the modelled cylinders experience drag by cross flow and

lift by flow parallel to the longitudinal cylinder axis. In the latter case, however, the

complex zigzag geometry of the leg segments with positive and negative inclination

results in a small overall angle of attack (Fig. 2B). In addition, peak lift coefficient of

cylinders is only 11-20% of the maximum drag coefficient at Reynolds numbers

between 7 and 20 (Babu and Mahesh, 2008; Vakil and Green, 2009). Together, this

results in at least 35-times less instantaneous lift than drag for the example in figure 2.

Thus, we did not further consider lift-induced moments. We determined aerodynamic

drag using a combined approach, in which we estimated the aerodynamic effective,

local frontal area with respect to the oncoming flow, the local air flow vector from

kinematic reconstruction, an experimentally validated, velocity- and thus Reynolds

number-dependent drag coefficient, and equation 7. Drag was estimated separately for

each leg and body segment, modelling each segment as a solid, rigid cylinder with

appropriate mean length, width and total mass. For cylinders, White (White, 1974)

suggested a Reynolds number-based, empirical approximation of drag coefficient, CD,

written as:

3/2Re101 DC , (4)

in which Reynolds number, Re, is derived from the local velocity of the segment and

a characteristic length of twice the cylinder radius. Compared to previous

measurements of drag coefficients on cylinders at cross wind, equation 4 yields

negligible differences at Reynolds numbers between 1 to 10 (Tritton, 1959). Mean

Reynolds number of a single leg is approximately 1.9 (235 mm s-1 body velocity,

cylinder radius 60m, 15 x 10-6 m2 s-1 kinematic viscosity). Besides body motion,

local velocity also depends on induced velocity (uind, downwash) generated by wing

flapping, which was calculated by (Ellington, 1984e; Usherwood and Lehmann,

2008):

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A

Luind

2 , (5)

with L, body lift, , the density of air, and A, the area covered by the beating wings

(Fig. 11D). We approximated the latter measure using a mean wing beat amplitude of

162 degrees and wing length of 2.5 mm (Lehmann and Dickinson, 1998).

Instantaneous body lift was calculated from translational movements of the fly and a

previously derived vertical damping coefficient Cvert of 54.8 mg s−1, written as:

gmtuCtumtL bvertvertvertb )()()( , (6)

with mb, the body mass, g, the gravitational constant, and t, the time.

To experimentally validate Reynolds number-dependent drag, we determined drag on

body appendages in a 1.05 m s−1 laminar flow wind tunnel and using a laser balance

(Lehmann and Dickinson, 1998). Reynolds number varied between approximately 2.8

(tarsi) and 8.3 (femur, Table 2). We removed hind legs from female flies, mounted

them on a flat surface and dried them over night in a stretched position to avoid

changes in leg posture owing to joint flexing. We mounted 7 legs at equal distance of

1.0 mm, orthogonal to a tungsten wire (127 µm diameter), positioned the wire with

the legs normal to the air flow, and rotated it to measure the legs’ drag at various

angles of attack (Fig. 11). Similar to this procedure, we estimated yaw angle

dependent body drag on a fly trunk, removing wings and legs from a dead animal and

gluing the trunk with the longitudinal body axis oriented normally to the wire. Drag

components owing to the tungsten wire were subtracted from the measures. We

subsequently fitted a sinusoidal curve to the data and estimated the drag coefficient,

CD, using fitted values and equation,

Su

DC

leg

D 2

2

, (7)

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in which D is drag, uleg the velocity at the centre of area of the body segment and S the

frontal surface area of the segment facing the flow. At 90 deg angle of attack at which

the flow is normal to hind legs and body trunk, drag coefficients were 2.96 and 1.41,

respectively. For comparison, drag coefficients for hind leg and body trunk, modelled

as a simple cylinder in equation 4 at comparable Reynolds number, are 3.14 and 1.44,

respectively, which is only 2-6% higher than the measured coefficients (Fig. 11).

Estimation of moments

Moments around the fly’s yaw, pitch, and roll axes rely on at least four, independently

acting mechanisms: moments owing to (i) lift (perpendicular) and drag (parallel to

relative flow) produced by the flapping wings, (ii) drag on hind legs, thorax, head and

abdomen, (iii) changes in moment arm of wing flapping-induced moments by

positional alterations of the fly’s centre of mass, and (iv) active mass movements of

hind legs and abdomen. In contrast to drag- and mass shift-induced moments, we

derived wing-induced moments from (i) instantaneous measures of body motion, (ii)

moments of inertia, I, with the body and hind leg segments approximated as cylinders

(see equations 10–15) and (iii) previously derived damping coefficients of the wings

for body rotation in freely flying fruit flies, Crot. These coefficients are 176 ng m2 s−1

for yaw, 204 ng m2 s−1 for pitch, and 352 ng m2 s−1 for roll (N = 170 flies, Shishkin

et al., 2012). Total instantaneous moment, MM, around the centre of body mass thus

equals:

)()()( tCtItM rotM , (8)

with the rotational, angular velocity of the animal (Hesselberg and Lehmann, 2007).

Wing-induced moments, MW, for posture and heading control were then calculated by

subtraction of the remaining components, which may be written as:

)()()()()()( tMtMtMtMtMtM DWISDMW , (9)

with MD body drag-, MS, body mass shift-, MI, inertia-, and MDW downwash-induced

moments. Downwash-induced moments were only considered in the analysis shown

in Fig. 7.

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We calculated drag-induced moments, MD, from the x-, y-, and z- components of the

cross product between drag on body, abdomen and hind legs using equation 7 and the

corresponding moment arm. The local flow vector was derived from the vector sum of

body motion, active leg motion, and, in cases in which we considered downwash,

from induced velocity and instantaneous body lift in equations 5 and 6, respectively.

To derive abdomen-induced mass shift moments, MS, we used a simplified approach:

we defined the moment arm as the distance between the aerodynamic centre of force

of each wing at 56% wing length and the fly’s centre of body mass (Ramamurti and

Sandberg, 2007). Since this distance varies throughout the wing flapping cycle, we

used a mean moment arm at mid half stroke, when the wing’s longitudinal axis was in

the horizontal and normal to the fly’s longitudinal body axis (Fig. 2C). At mid half

stroke and zero vertical body velocity, instantaneous vertical lift of each wing equals

to approximately 6.18 N (mb = 1.26 mg, cf. equation 6), and instantaneous horizontal

drag of each wing is approximately 2.25 N. We calculated the latter value according

to (i) the drag characteristics of a robotic fruit fly wing (Re = 134, Dickinson et al.,

1999), (ii) mean wing velocity at the wing’s centre of force, (iii) mean wing area of

one wing of 1.74 mm2 of Drosophila, and (iv) employing equation 7.

Estimation of moments of inertia

We estimated the fly’s moments of inertia by modelling the body trunk including fore

legs, middle legs and the hind leg coxae and femurs as a single object, composed of

solid cylinders (Fig. 8A). By contrast, hind leg tibia and tarsi were modelled

separately as moving cylinders with appropriate mass. Total mass of a hind leg was

9.42 ± 2.85 g (N = 4 groups of 10 legs each) and mean mass for hind leg coxa,

femur, tibia and tarsi was 2.02, 5.02, 1.72, and 0.66 g, respectively (Table 2). The

yaw, pitch, and roll moments of inertia of the cylinder were derived from the

following equation:

,coscoscos 2222 dmIIII cylccbbaa (10)

with Ia, Ib, and Ic the cylinder’s principal moments of inertia, a, b, and c the angles

between the axis of rotation and the cylinder’s principal axes a, b, and c, respectively,

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mcyl the cylinder mass, and d the distance between the centre of mass and the axis of

rotation (parallel axis theorem). We may write the principal moments of inertia for the

principal axis a thus as:

,2

2rmI

cyl

a

(11)

and for the axes b and c as:

,124

22 lmrmII

cylcyl

cb

(12)

with r the cylinder radius and l the cylinder length. The yaw and roll axes of the body

trunk cylinder are within the ac-plane and the pitch axis is the cylinder b-axis. This

leads to the following set of equations for moments of inertia of the body trunk about

the three axes, i.e. for yaw:

12

cos4

sin1

2

2

2

2lmrm

Icylcyl

yaw

, (13)

for pitch:

124

22 lmrmI

cylcyl

pitch

, (14)

and for roll:

,12

sin4

cos1

2

2

2

2lmrm

Icylcyl

roll

(15)

with the pitch angle. Eventually, to achieve total moments of inertia, we calculated

the moments of inertia for hind leg tibia and tarsi using equation 10 and added these

values to the moments of inertia of the body trunk.

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Statistics

If not mentioned otherwise, all data are given as means ± standard deviations

throughout the manuscript.

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Tables

Table 1. Mean and maximum turning moments in freely manoeuvring fruit flies. Data are absolute values, ignoring sign and thus direction of moments. Mean

performance ± standard deviation was calculated from 81 flight sequences and

maximum performance ± standard deviation from means of the 1% largest values

of each flight sequence. Value in parentheses is the mean of the 1% largest values

of all sequences (N = 24,596 data samples). MM, total moment derived from body

motion; MD, sum of moments induced by drag on body trunk and hind legs; MS,

moment induced by displacement of the fly’s centre of body mass; MI, moment

owing to inertia of hind legs and abdomen; MD,Leg, moment induced by drag on

hind legs; MD,Abd, moment induced by drag on abdomen; MD,Trans, drag-induced

total moment owing to translational body motion; MD,Rot, drag-induced total

moment owing to rotational body motion; MD,Act, drag-induced moment owing to

active movements of abdomen and hind legs relative to fly thorax.

Moment Axis Mean

performance

Maximum

performance

MM

(nN m)

Yaw 1.65 ± 1.00 6.29 ± 3.18 (11.2)

Pitch 2.23 ± 0.81 7.68 ± 2.73 (10.2)

Roll 3.13 ± 3.41 9.63 ± 8.92 (26.8)

MD

(nN m)

Yaw 0.03 ± 0.03 0.06 ± 0.05 (0.16)

Pitch 0.10 ± 0.08 0.16 ± 0.10 (0.36)

Roll 0.04 ± 0.04 0.07 ± 0.06 (0.22)

MS

(nN m)

Yaw 0.21 ± 0.39 0.46 ± 0.62 (2.55)

Roll 0.83 ± 0.72 1.52 ± 0.98 (3.81)

MI

(nN m)

Yaw 0.16 ± 0.05 0.82 ± 0.41 (0.96)

Pitch 0.34 ± 0.10 1.52 ± 0.65 (1.84)

Roll 0.15 ± 0.08 0.84 ± 0.68 (1.29)

MD,Leg

(pN m)

Yaw 35.2 ± 30.9 63.5 ± 52.1 (171)

Pitch 106 ± 85.9 159 ± 98.4 (386)

Roll 41.2 ± 39.2 74.0 ± 58.7 (220)

MD,Abd

(pN m)

Yaw 5.72 ± 5.92 14.7 ± 12.9 (39.7)

Pitch 13.6 ± 11.2 31.0 ± 19.5 (68.6)

Roll 4.73 ± 5.56 13.8 ± 14.5 (40.8)

MD,Trans

(pN m)

Yaw 19.6 ± 22.8 36.1 ± 40.5 (133)

Pitch 47.0 ± 54.8 78.9 ± 86.9 (324)

Roll 16.9 ± 22.5 31.2 ± 38.2 (150)

MD,Rot

(pN m)

Yaw 0.39 ± 0.93 1.92 ± 3.58 (9.38)

Pitch 0.39 ± 0.73 1.89 ± 3.34 (8.30)

Roll 0.55 ± 1.68 2.72 ± 7.14 (16.3)

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MD,Act

(pN m)

Yaw 0.05 ± 0.14 0.57 ± 1.88 (1.45)

Pitch 0.09 ± 0.15 0.92 ± 2.44 (2.18)

Roll 0.04 ± 0.08 0.44 ± 1.28 (1.92)

Table 2. Morphological measures of Drosophila hind leg segments. Mass of

each leg segment was estimated from its relative volume, determined from 21

blade elements and using total leg mass. The position of centre of mass is given as

a fraction of segment length with 0% the proximal segment end. Mean ± standard

deviation, N = 11 hind legs from 11 flies.

Coxa Femur-Trochanter Tibia Tarsus

Length (µm) 249 ± 30.4 635 ± 24.3 659 ± 23.3 742 ± 25.6

Mean width (µm) 120 ± 9.16 119 ± 5.18 68.3 ± 2.78 39.8 ± 1.88

Area (x103 µm2) 29.9 ± 5.12 75.4 ± 4.76 45.0 ± 2.91 29.5 ± 1.83

Volume (x106 µm3) 3.02 ± 0.75 7.21 ± 0.74 2.51 ± 0.25 1.01 ± 0.11

Centre of mass (%) 38.5 ± 2.24 44.5 ± 1.32 58.6 ± 1.02 34.8 ± 2.63

Centre of area (%) 43.9 ± 1.29 47.0 ± 0.79 54.8 ± 0.64 42.1 ± 1.42

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Figures

Fig. 1. Body posture, hind leg extension, and abdomen bending in freely

manoeuvring fruit flies. (A) Yaw, pitch and roll angles and corresponding axes. The

three-dimensional, instantaneous position of the centre of body mass, COMB, was

reconstructed from the centre of abdominal mass, COMA, and the centre of the

combined mass of thorax, head, and legs, COMT. V , deflection angle of the

abdomen in the vertical; , body pitch angle. (B) Top view (x-y plane) on a flight

path of a single fly cruising freely. Open dots indicate the centre of body mass every

10 ms, i.e. equal to every 35th recorded video frame, and attached lines show the

orientation of the animal’s longitudinal body axis. (C) Yaw, pitch, and roll angles

plotted for the flight sequence shown in B. Yaw angles are scored with respect to the

external coordinate system. Angles around the roll axis to the right (clockwise) and

upward pitching are positive. (D–F) Abdominal deflection and leg extension angles in

the vertical ( V , V ) and horizontal ( H , H ) for sequence in B, respectively.

Normalized angle histograms of all flight sequences are shown on the right in

respective colours. Data were derived from 7.03 s total flight time. N = 81 flight

sequences recorded in 14 flies.

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Fig. 2. Instantaneous moments of a fruit fly flying freely inside a random dot

flight arena. (A) Side view (y-z plane) of a flight path with superimposed body

positions (open dot) plotted every 11.4 ms during the 142 ms sequence. Time trace

shows the ratio between the sum of body appendage-induced (numerator) and wing

flapping-induced moments (denominator). Data are clipped at ±1. (B) Draft

illustrating moments induced by aerodynamic forces acting on the fly. (C) Draft

illustrating moments owing to body mass shift. (D) Total moment around the fly's

centre of mass (COMB) for sequence shown in A. (E) Yaw, (F) pitch, and (G) roll

moments around COMB. Positive values indicate clockwise moments around yaw,

pitch and roll axes (cf. Fig. 1A). Body drag-induced moment (MD, right axis) results

from aerodynamic drag on abdomen and hind legs, while inertia-induced moment (MI,

left axis) is estimated from relative movements of abdomen and hind legs. Mass shift-

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induced moment (MS, left axis) is the product of stroke cycle-averaged aerodynamic

force of each wing and the moment arm between the wing's centre of force at mid

down stroke and COMB. (H) Fractions of drag-induced moment owing to body

translation, MD,Trans, (I) body rotation, MD,Rot, and (J) self-induced moments owing to

active movement of abdomen and hind legs ("paddling") relative to thorax, MD,Act. (K)

Moment induced by the wings' mean downwash (induced flow), MDW. MDW is not

considered for total MD in E-G. See text and legend of table 1 for more information.

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Fig. 3. Difference of instantaneous drag-induced moments MD minus M*D around

COMB. Moments (M*D) for yaw (black), pitch (red), and roll (blue) were calculated

assuming static, non-moveable hind legs and abdomen, for the example shown in

figure 2. The difference highlights the contribution of active leg and abdomen

movements during manoeuvring flight. See text for more data.

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Fig. 4. Frequency of moment ratios during yaw (left column) and roll (right

column) steering in fruit flies. (A,B) Ratio between moments produced by body drag

(MD), mass shift (MS), inertia (MI), and active wing flapping (MW). (C,D) Moment

ratio of the 20 longest flight sequences (blue) with means in red (N = 81 sequences).

Unity (dotted) indicates similar contribution of active (wing flapping-induced) and

passive (abdomen- and leg-induced) moments to total response. Gray area highlights

the relative time at which rotational steering is dominated by leg and abdominal

control. (E,F) Mean moment ratio between drag- (red), mass shift- (grey), inertia-

(blue), and wing flapping-induced moments, respectively (N = 81 sequences).

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Fig. 5. Statistical analysis of coherence between the various components

underlying yaw moment control. (A–C) Coherence of moments of the five longest

recorded flight sequences (fly 1–5). Instantaneous coherence was scored as the

product of moment sign between wing flapping- and body drag-induced moment in A,

wing flapping- and mass shift-induced moments in B, and wing flapping- and inertia-

induced moments in C. Time-invariant relative frequency of coherence between the

two instantaneous moments of all recorded data is shown on the right, respectively,

with +(-) indicating equal (opposing) sign of moment. (D–F) Histogram of significant

correlation coefficients (Pearson test, N = 74 in D, N = 64 in E, N = 68 flight

sequences in F) obtained from linear regression analysis between moments shown by

the insets in A–C, respectively. A negative correlation coefficient suggests opposing

direction of moments. Means ± standard deviation.

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Fig. 6. Statistical analysis of coherence between the various components

underlying yaw moment. (A–F) Coherence was scored as the product of moment

sign between drag- and mass shift-induced moment in A and D, drag - and inertia-

induced moments in B and E, and mass shift - and inertia-induced moments in C and

F. See legend of Fig. 5 for a more detailed explanation.

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Fig. 7. Changes in moments caused by wing flapping-induced flow (downwash).

(A) Various time traces for drag-induced yaw moment (MD) of the flight sequence in

Fig. 2A with increasing consideration of downwash-induced moment MDW (colour-

coded; 0%, red; 100%, blue, 10% step width). (B-D) Moment means (red) ± standard

deviation (red area) and the 1% largest moments of all flight sequences (black, N =

24,596 samples of 81 flight sequences) plotted as a function of MDW. Mean yaw is

shown in B, mean pitch in C, and mean roll moment in D (N = 81 sequences). (E)

Ratio of drag- and wing flapping-induced yaw moments. For colour coding see A. (F)

Significance of downwash-induced yaw moments on correlation coefficients

calculated in Figs 5 and 6. Coefficients are shown for drag- (MD), mass shift- (MS),

inertia- (MI), and wing flapping-induced moments (MW). Asterisk indicates a

significant difference from zero (one sample t-test, p < 0.05). Mean standard

deviations are ±0.4 (red), ±0.6 (blue), and ±0.4 (green, N = 11).

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Fig. 8. Alteration in moments of inertia owing to hind leg movements during

flight. (A) Moment of inertia of yaw, pitch and roll was determined from a solid

model cylinder with appropriate size and mass (cf. Materials and Methods section),

rotating around the three body axes (cf. Fig. 1A). (B) Moments of inertia calculated

for the model cylinder are plotted as a function of the cylinder’s pitching angle, with

zero the horizontal. (C–E) Relative increase in moments of inertia compared to B

owing to additional mass motion of tibia and tarsi of two hind legs. Binned moments

of inertia within 1.0 deg pitch angle (black) are plotted for yaw in C, pitch in D and

roll in E. Gray area indicates the minimum-maximum range within an one-degree-bin

and solid lines show a theoretical prediction of maximum increase based on selected

hind leg extension angles. See text for more detailed information.

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Fig. 9. Video reconstruction of body markers in freely flying fruit flies. (A) Free

flight arena (not to scale). HS, high speed camera; IR, infra-red light camera, L; infra-

red laser sheet; LED, ultra-violet light emitting diodes; P, starting platform. (B) Fly

with fluorescent markers (M1–M6). Note that the image shows an animal with

marked wings instead of marked hind legs. (C) Time series of video images recorded

by high speed camera HS2. Images show fluorescent markers with tagged positions of

the body (M1–5), abdomen (M6) and hind leg tarsi (L1,L2). The white blobs in the

lower part of the images result from fluorescent markers on the starting platform. The

time interval of 5.7 ms corresponds to 20 recorded frames of the video sequence.

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Fig. 10. Abdomen bending and hind leg extension angle during manoeuvring

flight. (A,B) Side view of a tethered fruit fly responding to the motion of a visual

stimulus. Fruit flies mainly steer by changes in wing motion, abdominal movement,

and by moving tibia and tarsi of the hind legs (red). (C,D) Horizontal and vertical leg

extension angles, H and V, were reconstructed from the three-dimensional position

of the distal tarsal segment and the angular position of the femur, respectively.

Abdominal deflection angles, H and V, were derived from the animal longitudinal

axis, the abdomen’s centre of mass, COMA, and the abdominal point of rotation, PTA

(cf. Fig. 1A). Mass of abdomen and hind legs, COML1–4, including the combined mass

of head and thorax were estimated from weight measurements and using an

elementary blade approach. PN, position of the neck connective in the sagittal plane.

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Fig. 11. Experimental evaluation of drag coefficients and wake velocity

distribution in Drosophila. (A, B) The coefficients were estimated from drag

measurements of isolated legs, and truncated corpses of fruit flies (without legs and

wings) mounted in a low Reynolds number, laminar wind tunnel. Drag (D, black) of a

single, stretched out hind leg in A and body trunk in B plotted as the function of angle

of attack, , with respect to the oncoming flow. Blue, sinusoidal fit to data; left, D =

−0.0043 + 0.292 |sin()|; right, D = 0.709 + 1.585 sin(). (C) Calculated drag of a

single leg during free flight with mean ζV = 105 degrees (N = 81 flight sequences, Fig.

10), and depending on ζH and the horizontal component of uleg. See figure 2 for leg

configuration and Material and Methods section for angles. Grey area indicates the

leg's active steering range (see figure 1E and F). Maximum relative change in drag

within the grey area is 9.5% for each uleg. (D) Wake velocities measured in the

parasagittal plane of a tethered flying fly. The snapshot shows wake and fly from

lateral and was recorded by digital particle image velocimetry as described in a

previous study (Lehmann, 2012). Note that velocity vectors close to the insect body

are reconstructed from surrounding vectors because of laser light reflections at the

fly's cuticle. Tethering (pitch) angle was approximately zero and induced flow is thus

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directed sideways. For comparison, vertical, lift-supporting, mean induced velocity

based on equation 5 was 0.64 ± 0.13 m s-1 and the 1% maximum in each sequence

0.69 ± 0.12 m s-1 (N = 81 flight sequences). (E) Induced velocity for the flight

sequence shown in figure 2. Raw data (black) indicate stroke cycle synchronous

alteration in velocity (arrows). Smoothed data are plotted in red.

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